This disclosure provides a method for converting CBD to a tetrahydrocannabinol featuring the use of cheap and non-toxic aluminum isopropoxide as a catalyst. The method comprises (a) providing a reaction mixture comprising a catalyst in an organic solvent, wherein the catalyst comprises aluminum isopropoxide; (b) adding a reagent comprising CBD to the reaction mixture; (c) mixing the reaction mixture and allowing a reaction for converting CBD to a tetrahydrocannabinol to occur for a predetermine period of time; (d) removing the catalyst by filtration upon the completion of the reaction; (e) removing the organic solvent; and (f) eluting the tetrahydrocannabinol from the organic phase.